The Assignee of the present invention manufactures three phase 400 Hz. electrical power generators used in airframes. A first type of electrical power generating system uses a constant speed drive transmission to convert a variable speed power takeoff from a propulsion engine into a constant speed shaft drive which drives a three phase alternator for producing alternating current for powering appliances in an airframe. These systems run at high rotational speeds, such as 12,000 rpm when a four pole configuration is used, and 24,000 rpm when a two pole configuration is used.
FIG. 1 illustrates a prior art technique of the Assignee for retaining the stator 10 of a three phase electrical power generator, such as an integrated drive generator, used in an airframe such as a two pole or four pole unit in a housing 12. The retention mechanism utilizes a bolt 14 which extends through an aperture 16 in the housing into a tapped, threaded hole 18 within the magnetically permeable laminations 20 of the stator 10. With electrical power generating systems having two or four poles, the overall circumference of the stator is such that approximately one inch of material is available within the bore 18 for receiving the bolt 14 to retain the stator 10 within the housing 12 without protruding into the slots in which the electrical windings are placed. A one inch depth such as that indicated by reference numeral 22 is more than adequate to retain the stator 10 against axial and rotational movement within the housing 12. For the conventional electrical power generating system which is driven by a constant speed drive, the retention scheme illustrated in FIG. 1 is more than adequate to hold the electrical power generator against axial and radial movement within the housing 12. It should be noted that multiple bolts 14 are disposed around the circumference of the housing 12 at axially spaced apart locations. Only one bolt 14 has been shown for purposes of illustration.
A newer design of electrical power generating system for airframes is known as a variable speed constant frequency (VSCF) system. In this system, a power takeoff from a propulsion engine is used to directly drive a multiple phase alternator having six or more poles. The three phase alternating current which is produced by a VSCF alternator, which varies in frequency in direct proportion to the rotational speed of the propulsion engine, is rectified by a three phase full wave rectifier. The resultant DC current is used to drive a three phase solid state inverter which produces three phase 400 Hz. electrical power for powering appliances on an airframe.
Minimizing of weight is of paramount importance in designing equipment for airframes including electrical power generating systems. In the aforementioned constant speed drive system, the use of two or four poles minimizes the overall diameter of the stator. As a result, the thickness 22 which is the maximum depth of penetration without penetrating into the slots may be of sufficient depth, such as one inch, without causing an inordinate increase in the weight of the stator 10 while producing a low reluctance path magnetically coupling the poles together. This results from the fact that the rotor is driven at the aforementioned high rotational velocities of 12,000 or 24,000 rpm which permits a large amount of electrical energy to be generated even though the overall weight of the stator is acceptably low with a thickness, such as one inch, which provides acceptable retention.
On the other hand, the thickness 22 of the stator in an alternator used in the VSCF system is much less as a consequence of the larger diameter required to accommodate six or more poles. In a constant speed drive, the relative thickness of one inch is necessary to provide a low reluctance magnetic path around 90.degree. or 180.degree. of the stator. On the other hand, in an alternator in a VSCF system, the six or more poles reduce the path length between the poles such that a low reluctance magnetic path between the poles may be produced with a much lesser thickness 22 such as one-quarter of an inch. A six or more pole three phase alternator used in a VSCF system works efficiently with the maximum depth 22 being less than one-half of an inch which permits the overall thickness of the stator 10 to be reduced from the greater thickness in a two or four pole constant speed drive system where the greater thickness of one inch is necessary to create the necessary low reluctance path between poles.
In a six or more pole generator in a VSCF system, the typical one-quarter inch thickness 22 to the slot has insufficient retention capability to prevent undesired rotational and axial movement with respect to the housing 12. The depth 22 in a six or more pole generator in a VSCF system is insufficient to provide adequate retention threads in the stator core 10. Moreover, the use of laminations in the stator core to eliminate eddy currents further weakens the core's retention capability to hold threads 18 of the fastener 14. The retention scheme of FIG. 1 when applied to three phase alternators having six or more poles utilized in VSCF systems is inadequate to preclude the possibility of the failure of the retention of the stator 10 within the housing 12.